1. Field of the Invention
The present invention relates generally to a liquid crystal display device in which a plurality of pixels arrayed substantially in a matrix are driven with polarities that are cyclically reversed, and more particularly to a liquid crystal display device in which a non-video signal and a video signal are cyclically written in each of liquid crystal pixels as a pixel voltage.
2. Description of the Related Art
In recent years, mobile products to which liquid crystal panels are built, such as small-sized game machines, portable PCs and mobile phones, have rapidly gained popularity.
In general, the liquid crystal display panel is configured such that a liquid crystal layer is held between an array substrate and a counter-substrate. In the case where the liquid crystal display panel is of an active matrix type, the array substrate includes a plurality of pixel electrodes arrayed substantially in a matrix, a plurality of gate lines disposed along the rows of pixel electrodes, a plurality of source lines disposed along the columns of pixel electrodes, and a plurality of pixel switching elements which are disposed near intersections of the gate lines and source lines. The gate lines are connected to a gate driver which drives the gate lines. The source lines are connected to a source driver which drives the source lines. The gate driver and source driver are controlled by a control circuit. Each of the pixel switching elements is composed of, e.g., a thin-film transistor (TFT). When the associated gate line is driven by the gate driver, the pixel switching element is made conductive, thereby applying a pixel voltage, which is set on the associated source line by the source driver, to the associated pixel electrode. The counter-substrate is provided with a common electrode which is opposed to the pixel electrodes disposed on the array substrate. A liquid crystal pixel is constituted by a pair of each pixel electrode and the common electrode, together with a pixel region which is a part of the liquid crystal layer located between these paired electrodes. A driving voltage for the pixel is a difference between a pixel voltage, which is applied to the pixel electrode, and a common voltage which is applied to the common electrode. Even after the pixel switching element is turned off, the driving voltage is held between the pixel electrode and the common electrode. The alignment state of liquid crystal molecules in the pixel region is set by an electric field obtained between the electrodes to control the transmittance of the pixel. The polarity reversal of the driving voltage is executed, for example, by cyclically reversing the polarity of the pixel voltage with the common voltage used as a reference. Thus, the direction of electric field is reversed to prevent non-uniform distribution of liquid crystal molecules in the liquid crystal layer.
In mobile products, the power consumption of a backlight, driving circuits, etc., needs to be reduced in order to enable long-time battery-powered operations. On the other hand, in the case of a product that uses a low response-speed liquid crystal such as a TN liquid crystal, a moving image blurs when it is viewed, and a good moving-image viewablity cannot be obtained. It is thus required to improve the display quality, as well as reducing the power consumption.
In the field of large-sized liquid crystal TVs, liquid crystal display panels of an optically compensated bend (OCB) mode, which has a high liquid crystal responsivity that is needed for displaying moving images, have begun to be adopted. This liquid crystal display panel performs a display operation by transitioning the alignment state of liquid crystal molecules from a splay alignment to a bend alignment in advance. In this case, the bend alignment tends to be reversely transitioned to the splay alignment if a voltage-non-applied state or a nearly voltage-non-applied state continues for a long time. In this type of liquid crystal display panel, black insertion driving is used to prevent the reverse transition to the splay alignment (see Jpn. Pat. Appln. KOKAI Publication No. 2002-202491). In this case, the liquid crystal display panel is driven so as to perform video signal display, for example, in about 80% of 1 frame period, and to perform black display (non-video signal display), with which a driving voltage is set at a maximum value, in the other about 20% of the 1 frame period. Since the black insertion driving provides a pseudo-impulse response of luminance in moving image display, like a CRT, the black insertion driving is effective in clearing retinal persistence occurring on a viewer's vision, thus making the movement of an object appear smoother. Therefore, the black insertion driving has attracted attention as a technique which remarkably improves the moving image viewablity.
For example, in the liquid crystal display panel in which the black insertion driving is executed, two write operations, that is, a black insertion write operation and a video signal write operation, are executed in 1 frame period in order to apply a pixel voltage to each of pixel electrodes. FIG. 15 shows timings of sequentially driving gate lines Y1, Y2, Y3, . . . , for black insertion writing and video signal writing. In this example of black insertion driving, the gate driver sequentially drives, as black insertion scanning, the gate lines Y1, Y2, Y3, . . . , by making use of a first half of 1 horizontal scanning period (1H) of a video signal, and further sequentially drives, as signal write scanning, the gate lines Y1, Y2, Y3, . . . , by making use of a second half of the 1 horizontal scanning period (1H) of the video signal. In the first half of the 1 horizontal scanning period (1H), the source driver drives all the source lines so that pixel voltages Vs for black insertion may be written in pixels for one line. In the second half of the 1 horizontal scanning period (1H), the source driver further drives all the source lines so that pixel voltages of the video signal may be written in pixels for another one line. In this case, the black insertion period for each pixel is equal to a period between the black insertion scanning and the signal write scanning.
The power consumption in the source driver is now considered. FIG. 16 shows a scanning diagram and a waveform of a source line potential obtained in a prior-art driving in which black insertion is not performed. FIG. 17 shows a scanning diagram and a waveform of a source line potential obtained in a prior-art driving in which black insertion is performed. The ordinate of the scanning diagram indicates a vertical scanning position corresponding to a gate line, which is driven in the liquid crystal display panel, and the abscissa indicates a time as a scanning timing at the vertical scanning position.
For example, in the case where a dot-reversal (or line-reversal) drive scheme is applied to the liquid crystal display panel, the source line is set at a potential corresponding to a pixel voltage which is reversed in polarity in every 1 horizontal scanning period (1H), for example. Since the source driver charges the source line in every 1 horizontal scanning period with a polarity which is reversed with the common voltage used as a reference, the power consumption, which is calculated by integrating the charge current on the time axis, is very high. If the dot-reversal (or line-reversal) drive scheme is changed to a column-reversal (or frame-reversal) drive scheme, the source driver charges the source line with a reverse polarity in every 1 frame period. The 1 frame period is much longer than the 1 horizontal scanning period. Thus, the power consumption, which is calculated by integrating the charge current on the time axis, is greatly reduced than in the dot-reversal (or line-reversal) drive scheme.
On the other hand, if black insertion is performed in the dot-reversal (or line-reversal) drive scheme, the source driver charges the source line with a reverse polarity in every 1 horizontal scanning period, as in the above-described case. Since the pixel voltage is set at a polarity which is reversed with the common voltage used as a reference in every 1 horizontal scanning period, the power consumption, which is calculated by integrating the charge current on the time axis, is very high. If black insertion is performed in the column-reversal (or frame-reversal) drive scheme, the pixel voltage is set at a reverse polarity in every 1 frame period, which is much longer than 1 horizontal scanning period. In particular, in a time period in which black insertion scanning and signal write scanning overlap, the pixel voltage shifts between a black level and a video level with a polarity which is reversed with the common voltage used as a reference in every 1 horizontal scanning period/2. Thus, unlike the case in which black insertion is not performed in the column-reversal (or frame-reversal) drive scheme, the power consumption, which is calculated by integrating the charge current on the time axis, cannot be greatly reduced.
As is clear from the above, in the case where the black insertion drive scheme is to be adopted in order to improve the moving image viewablity, it is easy to apply the black insertion drive scheme to products, such as large-sized liquid crystal TVs, in which restrictive conditions relating to power consumption are relaxed, but it is difficult to apply the black insertion drive scheme to mobile products in which restrictive conditions relating to power consumption are severe.